1. Field of the Invention
The present invention relates to a method and an apparatus for fabricating solar-grade high purity powder compacts using fine polycrystalline silicon powders without addition of binders, as well as a binder-free silicon compact fabricated by the same.
2. Description of the Related Art
The global solar cell market has been growing strongly by annual average growth of over 35% since the mid-1990s wherein crystalline silicon-based solar cells account for more than 97% of solar cells in the market in respect of material and take the initiative in market growth. In the meantime, high purity silicon material which is a core material for solar cells has been supplied by scraps of single crystalline silicon for semiconductor wafers or off-specification polycrystalline silicon. However, the global solar cell market has been rapidly growing since the mid-1990s and the supply did not fall short of the fast growing demand with the entrance toward the industrialized phase and consequently the solar cell market has reached the situation of serious supply shortage since 2004. Shortage phenomenon of silicon material will be continued worldwide for some time due to the demand for investment in facilities on a large scale in order to manufacture the polycrystalline silicon raw material. In particular for example, Korea depends on imports for almost all of demand and Korea is now trying to prepare for its own solution such as the construction of a silicon factory with 3,000 ton annual production capacity by DC Chemical Co., Ltd from 2006, to promote national solar cell industry and obtain the international competitiveness.
In the present day, worldwide high purity polycrystalline silicon is manufactured by purifying a raw material silane gas including silicon (trichlorosilane or mono-silane) and then precipitating a purified silane gas into a high purity dense polycrystalline silicon in a high temperature. At this time, a large amount of fine polycrystalline powders are produced as byproducts in the process of silane decomposition. Accordingly, the necessity for utilizing the fine polycrystalline silicon powders as the material for solar cells is being raised.
Upon examining closely attempts to use such polycrystalline silicon powders as the material for solar cells, you will find out that the Siemens and the Freiburg in Germany made researches to fabricate wafers directly from polycrystalline silicon powders using a sintering method in the past 1980s in order to reduce a manufacturing process and material loss. However, since this method was considered as not having merits, compared to the commercial processes, it did not reach commercialization. Until now the polycrystalline silicon powder, mixed with high purity raw materials, has been used mainly as a charge material of HEM process. Recently, methods of manufacturing a pellet-type compact from polycrystalline silicon powder to overcome problems involved in low capacity and handling have been proposed. To classify them largely, there are methods of manufacturing compacts with addition of chemical additives such as binders, or manufacturing compacts under a room temperature or through a melting process without using chemical additives such as binders. However, in view of research experience and viewpoint of inventors of this invention, there is a problem of containment due to remaining of chemical additives in the case of using chemical additives, there is a problem of the limitation of density and strength of compacts in the case of manufacturing under a room temperature without binders and there is a problem of being the possibility of pollution due to the passage of the melting process in the case of manufacturing through the melting process without binders.
Furthermore, there occurs only in Korea more than 100 ton per year of saw dust sludge during the manufacturing processes of semiconductors and solar cells but the sludge is now used as low level products (1˜5$/kg).
Therefore, technology development of recycling scraps and saw dust sludge into low priced silicon raw material for solar cells is urgently needed in reality.
In “Technology of Compacting and Melting and Casting polycrystalline silicon fine powders for economical Si ingot production”, B. M. Moon et al., published on Proceedings of The Korean Society for New and Renewable Energy on Jun. 21, 2007, a lot of fine powders produced in the process of manufacturing high purity polycrystalline silicon are washed and dried, 1.6 g to 1.8 g of the fine powders were loaded in a die without the addition of binders, the system including the die filled with fine powders was first evacuated to 3 torr and the fine powders are then compacted with a uniaxial pressure of 700 kgf/cm2 in the room temperature. As a result, it is presented that polycrystalline silicon powder compacts can be obtained even without using binders.
In the case of a compact specimen using the washed powders without addition of binders, the electrical resistivity of the compact specimen does not reach electrical resistivity 0.5 Ωcm which is generally required for wafers of solar cells. This is why byproducts produced in a wet washing process and contaminants contained in wash liquid itself are not removed completely.
At the time of silicon crystal growth, in order to prevent containments such as water and oxides from mixing due to a large surface area of polycrystalline silicon powders, the polycrystalline silicon powders were dried in a vacuum atmosphere or were subjected to a dry heat-treatment in a reducing atmosphere of 10% H2—Ar in the temperature of 1,200˜1,300° C., for one hour. The powders were then melted and fabricated to be silicon crystals. For two silicon crystalline powder compacts made in the above method, the electrical resistivity thereof was 3 Ωcm and 4.6 Ωcm, respectively.
However, the process for compacting stably and effectively fine powders generated a lot as by-products in the current process to precipitate high purity dense polycrystalline and powder scraps generated in the semiconductor process, was not proposed in this research.
In addition, since powders or scraps were low in a bulk density at the time of loading, melting efficiency was low and it was hardly possible to manufacture a large mass of ingot, it is difficult to handle and deliver powders or scraps and there was a problem that the contaminants are produced at the time of delivery.